Fire-resistance limits R15, R30, R60, R90: what they mean and how to choose

When it comes to fire protection of steel, a client sooner or later runs into a mysterious marking: R15, R30, R45, R60, R90, R120. The designer specifies it in the brief, the inspector checks the site against it, and the paint supplier promises to «reach» the required value. Let us figure out what these numbers actually mean.

What a fire-resistance limit is

The letter R is the ability of a load-bearing structure to keep its strength and stability under fire conditions (from the French résistance). The number next to it is time in minutes during which the element is guaranteed to «hold» the load under the standard fire temperature curve. R30 means a beam or column will not lose its load-bearing capacity for at least 30 minutes. R90 — an hour and a half.

Why is this critical specifically for steel? Metal does not burn, and therein lies its treachery. An unprotected steel structure behaves predictably badly in a fire: already at 500 °C steel loses about half of its strength, and at 600 °C about 70 %. Open flame drives the structure to these temperatures within 10–15 minutes. Then the column «flows», the truss sags, and the building folds even before the firefighters arrive. The task of fire protection is not to «save» the metal, but to win those very minutes: for evacuating people and for the crews to work.

What the required limit depends on

The required R value is not picked out of thin air — it is set by the designer based on the building’s responsibility class, number of storeys, purpose and the function of the structure itself. The general logic is:

  • R15–R30 — typical for single-storey production, warehouses, retail pavilions, secondary elements.
  • R45–R60 — multi-storey public and industrial buildings, the load-bearing frame.
  • R90–R120 — buildings of increased responsibility, high-rise construction, key load-bearing elements on which the stability of the whole structure depends.

In Uzbekistan the exact requirements are set by building codes (ShNK 2.01.02 and the relevant fire-safety documents). Improvisation is not allowed here: understate the limit and you risk failing acceptance and, more importantly, people’s lives.

How paint provides the required R

The thin-film fireproof coatings we use (the TERM range) work on the intumescent principle. When heated, the paint foams up and forms a porous heat-insulating layer — a char dozens of times thicker than the original coating. This «armour» slows down the heating of the steel. More about the materials on the fireproof paints page, as well as in the TERM-001, TERM-007 and TERM-009 cards.

A key point that is often missed: one and the same composition can give R30, R60 and R90 — the difference is in the applied thickness. The higher the required limit, the thicker the calculated layer. Moreover, the thickness also depends on the massiveness of the structure — its section factor. A thin-walled profile heats up faster than a massive column and requires a thicker paint layer for the same result. That is why the calculation is always individual.

What this means for the client

  1. Do not order «just fire protection» — specify the required R limit for each type of structure in the design. Without this figure any calculation is meaningless.
  2. Require from the contractor a thickness calculation for the specific R and your structure, not «by eye». Saving on thickness is a direct understatement of the actual fire-resistance limit.
  3. Check that the composition is certified for exactly the declared limit. The test report is the only honest confirmation that R30 is R30 and not marketing.

STATERM selects TERM fireproof compositions for the required fire-resistance limit, calculates the thickness for your structure and provides application with quality control. Whether you need R15 for a warehouse or R90 for a load-bearing frame — we will help choose a solution and pass acceptance.

Call: +998 90 918 81 88.

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